In a number of applications it would be desirable to be able to provide a relatively stationary high altitude platform, hence the desirability of the present invention.
One known kind of stationary high altitude platform is a geo-stationary satellite located 36,000 km above the earth. While a geostationary satellite system may have a large “footprint” for communications or surveillance purposes, this may be higher than is desirable for high resolution observation, and the development and launch cost of a spacecraft may tend to be very high. Non-stationary, or low orbit satellites are also known, but they are at any given point in the sky only momentarily. It would therefore be advantageous to be able to operate a stationary platform at lower altitude, lower complexity, and rather lower cost.
A number of concepts for high atmospheric altitude platforms already exist, such as high altitude balloons, large dirigibles or blimps, unmanned heavier-that-air aircraft (drones) of traditional configuration or of flying wings configuration. Free balloons or tethered balloons would not tend to be suitable: a free balloon is not tethered, and will tend not to stay in one place; a 40,000–60,000 ft tether is not practicable (a) because of the weight of the tethers themselves; and (b) because of the danger to aerial navigation. Heavier-than-air aircraft tend not to have the required endurance, and any aircraft that relies on airflow over a lifting or other control surface must maintain sufficient velocity to maintain control, a problem that worsens when the density of the atmosphere is reduced.
Traditional airships, whether blimps or having a rigid internal skeleton tend generally to be low altitude aircraft, seldom being used at altitudes above about 5,000 ft above mean sea level. Modern airships that rely on the buoyancy of a lifting gas may tend to suffer from a number of disadvantages, such as (a) poor low-speed manoeuvrability; (b) the need for relatively large ground-crews for take-offs and landings; (c) the need for relatively large fields from which to operate; (d) complicated and expensive infrastructure for mooring (parking); and (e) susceptibility to damage in turbulent atmospheric conditions. In the view of the present inventor, many, if not all of these disadvantages appear to stem from the fundamental shape and configuration of traditional airships—that is, the characteristic elongated, finned hull.
The manoeuvrability of traditional airships tends to be related to the design and structure of their fins and control surfaces. Below 10 to 15 km/h (6–10 mph), there tends no longer to be sufficient airflow over the fins' control surfaces, making them ineffectual. When the pilot slows down, as when landing, a ground crew of up to 20 people may be required to assist the pilot. The same size of crew may also be required for take-off.
The spherical airship described herein has double envelopes. The outer envelope is load bearing and the inner envelope contains the lifting gas. For normal low-level flights at take-off, the inner envelope may typically be filled to 80%, of the internal volume of the outer envelope allowing the lifting gas to expand with altitude or temperature changes or both. When the inner envelope is fully expanded, the airship is at pressure altitude; meaning that it cannot climb higher without valving some lifting gas.
In the presently described airship, the air inside the outer envelope is slightly pressurized by electric blowers to maintain the airship's generally spherical shape and to resist deformation from wind loads. For the high altitude airship of the present invention, operating at 60–70,000 ft., the envelope must be sufficiently large enough to accommodate the 1,600–1,700% lifting gas expansion. Accordingly, in the present invention, at lift-off, the inner envelope may be filled to only as little as 1/18 of its total volume. The remaining 17/18 are filled with air at a slight (over) pressure.
During the climb to altitude, the lifting gas will tend to expand adiabatically, eventually occupying approximately 16/18ths of the total volume. At the designed operational altitude, it is intended still to have enough space to expand with temperature increase during daytime sun exposure. Note that the spherical airship tends not to have balancing problems at any stage of “fullness”. The weight of the payload is at the bottom central portion of the airship, and the lift is directly above this with all the gravity and buoyancy forces acting straight up and down.
Traditional cigar shaped blimps may also tend to present other disadvantages when viewed in the context of an aircraft having a high altitude service ceiling. Conventionally, cigar shaped airships employ fore and aft balloonets that can be inflated, or deflated, as the internal gas bags expand or contract with changes in altitude or temperature. Differential inflation of the balloonets can also be used to adjust airship trim. The balloonet operation between sea level (where ambient pressure is about 14.7 psia) and 5000 ft (where ambient pressure is about 12.5 psia) may involve balloonets of roughly 20% of the internal volume of the aircraft. To reach a service ceiling of about 60,000 ft (where the ambient pressure is about 1.0 psia), the volume of the lifting gas used at lift-off from sea level may be as little as about 1/18 of the volume of the lifting gas at 60,000 ft. This may present significant control challenges at low altitude for a cigar shaped aircraft. Further, conventional airships tend to rely on airflow over their control surfaces to manoeuvre in flight. However, at high altitude the density of the air is sufficiently low that a much higher velocity may be required to maintain the level of control achieved at lower altitude. Further still, blimps and dirigibles are known to be susceptible to “porpoising”. At 60,000 ft there is typically relatively little turbulence, and relatively light winds, or calm. In a light or “no-wind” situation, it may be difficult to maintain a cigar shaped dirigible “on station”, i.e., in a set location for which the variation in position is limited to a fixed range of deviation such as a target box 1 km square relative to a ground station. Although 1 km may seem like a large distance, it is comparatively small relative to an airship that may be 300 m in length.
By contrast, a spherical airship may have a number of advantages, some of which are described in my U.S. Pat. No. 5,294,076, which is incorporated herein by reference. A spherical airship is finless, and so therefore does not depend on a relatively high airspeed to maintain flight control. For example, when equipped with a propulsion system that has thrust deflectors (louvers) located in the propeller slipstream, steering and altitude control can be achieved through the use of varied and deflected thrust.
With equal thrust on both engines the airship can be flown in a straight line. Increasing (or decreasing) the thrust on one side causes the airship to turn. Deflecting the propwash downward may tend to cause the airship to ascend; deflecting the propwash upward may tend to cause the airship to descend. The prototype developed by the present inventor is highly manoeuvrable even at low speed or when hovering, and tends to be able to turn on a dime.
The present inventor has dispensed with a traditional external gondola, and has, in effect, placed the gondola inside the envelope, allowing a generally larger space for the pilot, passengers (as may be), and payloads, (as may be). Without an external gondola the spherical airship may tend to be capable of landing on, and taking off from, water. Landing procedures are comparatively uncomplicated.
A substantially spherical airship has the most efficient ratio of surface area to volume. This may tend to result in a relatively low leakage rate of the lifting gas. The spherical shape also tends to facilitate the spreading of the payload without unduly affecting the balance (pitch) of the aircraft.
The present inventor has noted that when a spherical object, such as a spherical airship, is propelled through an ambient fluid, such as air, the flow of the ambient about the spherical shape tends to have a separation point, beyond which the flow is turbulent. It would be advantageous to shift this separation point further toward the trailing portion of the aircraft, since this may tend to reduce drag.
The present inventor has also noted other properties of a spherical airship that may tend to make it suitable for relatively long endurance use as a communications or surveillance platform. First, the envelope may tend to be transparent to electro-magnetic waves in the frequency ranges of interest, namely the electronic communications frequencies. This may tend to permit (a) remote control of the platform from a ground station, further reducing the weight aloft and lessening both (i) the risk of human injury in the event of a machine failure; and (ii) the need to land frequently for the comfort of the crew; (b) the use of the platform as a communications relay station for sending and receiving signals; and (c) the use of the station as a radar platform or as a listening station. In addition, it may be desirable to be able to refuel a stationary airship at altitude, thus permitting extension of the duration of operation.